The present invention relates to a vehicular heat generator having a rotor and a viscous fluid contained in a heating chamber defined in a housing to generate heat by subjecting the viscous fluid to shearing by rotating the rotor, and to a viscous fluid for the same.
Japanese Unexamined Patent Publication No. HEI 2-246823 (corresponding to U.S. Pat. No. 4,993,377) discloses a vehicular heating system and a heat generator incorporated in the system. In the heat generator, shear force is applied by a labyrinth-shaped rotor to a viscous fluid sealed in a heating chamber defined in the heat generator to convert rotational energy of the rotor to thermal energy and generate heat as a result. The patent publication describes use of a silicone oil as an example of the viscous fluid. The Japanese Unexamined Patent Publication No. HEI 2-246823 recites: "The heat generating capacity of the heat generator can be preset as desired depending on the viscosity coefficient of the viscous fluid sealed in the heating chamber. In other words, the higher the viscosity coefficient of a viscous fluid, the greater the shear force occurring at grooves of the labyrinth and thus the higher the heat generating capacity is . . . On the other hand, the lower the viscosity coefficient of a viscous fluid is, the smaller the shear force occurring at the grooves of the labyrinth and thus the lower the heat generating capacity becomes."
The Japanese Unexamined Patent Publication No. HEI 2-246823 merely describes, referring to properties of the viscous fluid, the very basic concept that it is preferred to select a viscous fluid having a specific viscosity so as to set a desired shear heat generating capacity of the heat generator.
However, the present inventors found that a viscous fluid, employable in a vehicular heat generator designed to generate heat by shearing viscous fluid with a rotor, has a certain optimum range in its rheological properties. According to the finding of the present inventors, if a highly viscous fluid (typically a highly viscous silicone oil) is merely employed for the purpose of increasing heat generating capacity, the viscous fluid deteriorates soon due to the high heat thus generated and is thus unable to continue heat generation by mechanical shearing. Accordingly, it is not realistic to select the viscosity of a viscous fluid simply in view of improving the heat generating capacity.
Further, while vehicular heat generators are in many cases driven by power from vehicle engines, the engines involve a drawback in that they are operated in wide speed ranges, respectively, so that it is impossible to expect a constant power supply from each engine at a stable rotating speed. Accordingly, the rotating speed of the rotor is increased dramatically by the engine, when the engine speed is increased. However, the viscous fluid, having desired properties (particularly viscosity), is selected to obtain optimum and stable heat at a normal revolution speed. Therefore, over-shearing and over-heating may result. Overheating eventually deprives the viscous fluid of the desired properties, as described above.
It is an objective of the present invention to provide a viscous fluid that is employable in a vehicular heat generator based on studies and findings on the desired properties of viscous fluids for heat generators.
It is another objective of the present invention to clarify the relationship between the heating value generated by a heat generator or the time until the heating value is equilibrated and specific nominal viscosity values and to provide a vehicular heat generator employing a viscous fluid having a desired nominal viscosity.